Activation of glycogen phosphorylase in rat pheochromocytoma PC12 cells and isolated hepatocytes by organophosphates.
Several organophosphates including diisopropylfluorophosphonate (DPF) and a variety of compounds used as chemical warfare agents produced dose- and time-dependent increases in phosphorylase-a, the phosphorylated form of glycogen phosphorylase in rat pheochromocytoma cells, PC12, and isolated hepatocytes. Increases in phosphorylase-a did not occur in cells exposed to the carbamates, physostigmine or pyridostigmine, or to O-ethyl S-2-diisopropylaminoethylmethyl-phosphonathiolate (VX), an organophosphate which is protonated at physiological pH. When extracellular pH was increased to pH 8, VX acted like the other organophosphates and increased phosphorylase-a activity. The possibility that organophosphates increase phosphorylase-a in intact cells by releasing Ca2+ from intracellular binding sites is supported by the following findings: organophosphate-induced increases in phosphorylase-a did not correlate with changes in cyclic AMP in the two cell types studied; in PC12 cells, increases in this activity occurred in the absence of extracellular calcium and were not inhibited by the calcium channel blocker, verapamil; fluorescence of the calcium sensitive dye, Quin-2, in PC12 cells preloaded with the acetoxymethyl ester of the dye was increased by soman; finally, addition of the calcium ionophore, A23187, to PC12 cells maintained in calcium-free medium caused sarin-stimulated phosphorylase-a activity to return rapidly to basal levels. Collectively, these data argue strongly that organophosphates increase phosphorylase-a activity in intact cells via a novel mechanism involving release of calcium from intracellular binding sites.
Kauffman
F C
FC
Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.
Davis
L H
LH
Whittaker
M
M
D000227:Adenine Nucleotides / Q000032:analysis; D000818:Animals; D002118:Calcium / Q000032:analysis; D002478:Cells, Cultured / Q000187:drug effects; D003600:Cytosol / Q000032:analysis; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004789:Enzyme Activation / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000187:drug effects* / Q000201:enzymology; D008297:Male; D009943:Organophosphorus Compounds / Q000494:pharmacology*; D010673:Pheochromocytoma / Q000201:enzymology*; D010762:Phosphorylase a / Q000378:metabolism*; D006005:Phosphorylases / Q000378:metabolism*; D010830:Physostigmine / Q000494:pharmacology; D011729:Pyridostigmine Bromide / Q000494:pharmacology; D051381:Rats; D014407:Tumor Cells, Cultured / Q000187:drug effects; D014700:Verapamil / Q000494:pharmacology
D000227:Adenine Nucleotides; D009943:Organophosphorus Compounds; D010830:Physostigmine; D014700:Verapamil; D010762:Phosphorylase a; D006005:Phosphorylases; D011729:Pyridostigmine Bromide; D002118:Calcium